If there is a discount on that order, then you could add another table row tag and calculate the percentage. So it'd be something like this:

add_action('woocommerce_cart_totals_after_order_total', 'wc_discount_total', 99);

add_action('woocommerce_review_order_after_order_total', 'wc_discount_total', 99);

function wc_discount_total()
{
  global $woocommerce;
  $discount_total = 0;

  foreach ($woocommerce->cart->get_cart() as $cart_item_key => $values) {

    $_product = $values['data'];

    if ($_product->is_on_sale()) {
      $regular_price = $_product->get_regular_price();
      $sale_price = $_product->get_sale_price();
      $discount = ($regular_price - $sale_price) * $values['quantity'];
      $discount_total += $discount;
    }
  }
  if ($discount_total > 0) {
    echo '<tr class="cart-discount">
      <th>' . __('Your Savings', 'woocommerce') . '</th>
      <td data-title=" ' . __('You Saved', 'woocommerce') . ' ">'
      . wc_price($discount_total + $woocommerce->cart->discount_cart) . '</td>
      </tr>';

    $total = WC()->cart->cart_contents_total;
    $total_saved = wc_price($discount_total + $woocommerce->cart->discount_cart);
    $percentage_saved = round(($total_saved * 100) / $total);

    echo '<tr class="cart-percentage-discount">
      <th>' . __('Percentage Saved', 'woocommerce') . '</th>
      <td data-title=" ' . __('You Saved', 'woocommerce') . ' ">' . esc_html($percentage_saved . "%") . '</td>
      </tr>';
  }
}